The present invention relates to Raman amplification, and in particular to a method and apparatus for providing Raman amplification that is suitable for, but not limited to, providing Raman amplification in a telecommunications system.
Raman amplifiers use a non-linear effect called Raman scattering, whereby a small proportion of light at the pump frequency is scattered by the molecules in a medium (i.e. the optic fibre), and down shifted in frequency by an amount dependant on the vibrational modes of the medium. This down shifted light, called the Stokes wave, may have a fairly broad spectrum, the intensity of which is dependant on the fibre type and geometry. Stimulated Raman scattering (SRS) occurs when light at the same frequency as the Stokes wave is instant in the fibre, and results in the amplification of the incident light.
As the gain co-efficient is small, Raman amplifiers typically require long lengths of fibre (e.g. greater than 5 kilometers) to achieve satisfactory gain (e.g. greater than 20 dB). Distributed Raman amplification (Raman amplification in the transmission fibre) can thus be used to extend the reach of long-haul optical transmission systems by amplifying signals whilst they are in the transmission fibre, significantly proving the signal-spontaneous beat noise performance at the receiver. As the Raman pump power is increased, whilst the gain within the fibre also increases a noise effect called multi-path interference (MPI) can occur due to double Rayleigh scattering (DRS) of the signal.
FIG. 1 shows a short length of a fibre 10 in which DRS is occurring. The information signal light 12 is Rayleigh scattered 14 by small irregularities in the fibre. This results in a back scattered signal 16 travelling in the opposite direction to the information signal (i.e. it is counter propagating rather than co-propagating). This signal 16 may undergo Raman gain, and may be scattered again 18, resulting in a co-propagating DRS noise signal 20. Although the noise signal 20 has taken a different path than the information signal 12, and thus is likely to be incoherent, it does represent noise and can cause system penaltys. It is therefore desirable to limit the distance over which distributed Raman amplification is applied in a transmission fibre.
Relatively high power sources (e.g. 300 mW or greater) are used as Raman pumps. If such a signal is launched into an optical fibre, and the optical fibre broken, the resulting light output from the fibre could be exceptionally dangerous. Consequently, a shutdown mechanism is necessary. Conventional systems would rely upon the detection of the Raman pump signal at the end of the span (i.e at the next amplifying stage), with a signal being sent back via a supervisory system to the Raman pump source. Such a shutdown system has drawbacks. For instance, it requires the generation and transmission of a separate, supervisory signal. Also, due to the detection location at the end of the span, and the subsequent requirement to signal back to the Raman pump (typically at the start of the span) then the system is relatively slow. If the Raman pump is not located at the start of the span, then this can bring its own implementation problems e.g a separate power source is required for the Raman pump source.
It is an object of the present invention to address one or more of the problems of the prior art.
In one aspect, the present invention provides a system comprising a first optical waveguide suitable for transmitting a first information signal, a second optical waveguide suitable for transmitting a second information signal, and input means for providing an electro magnetic radiation signal into said first waveguide, said signal being suitable for providing Raman amplification of the information signals, the system further comprising an optical coupling between said waveguides, arranged to transmit at least a portion of the Raman amplification signal from the first waveguide to the second whilst substantially blocking the transmission of the information signals between the waveguides.
Consequently, such a system allows the sharing of a single Raman pump source between two optical waveguides carrying different information signals.
Preferably the system further comprises a detector means arranged to detect the transmission of the Raman amplification signal along the second waveguide. This allows a positive test that the Raman pump and the transmission fibre are functioning correctly.
For instance, said detector means can comprise an optical power detector, an optical tap coupling the second waveguide to the detector, and filtering means arranged to pass the Raman amplification signal but substantially block the transmission signals.
Preferably the system further comprises control means arranged to prohibit the provision of the Raman amplification signal into said first waveguide when said detector means does not detect the Raman amplification signal. This provides a safety mechanism in the event of a break in the optical path e.g. a fibre break.
The system may further comprise a source suitable for providing said Raman amplification signal.
Preferably, said detector means and said source are co-located. This allows the sharing of any of the power supply, control signals and control processor (hardware and/or software).
Preferably, said optical coupling comprises at least one of a dielectric coupler, an optical circulator, and a tapered fibre wavelength selective coupler.
Preferably the system further comprises an optical isolator arranged such so as to suppress multi-path interference. This may be provided in the form of an integral part of another component e.g. an optical circulator, or may be a separate component.
Preferably, the system is arranged such that the first information signal is substantially transmitted in the opposite direction to the second information signal, and the Raman amplification signal is transmitted along each respective waveguide in the opposite direction to the information signal for that waveguide. As the Raman amplification signal is counter propagating with respect to the information signals, the noise transfer between the Raman amplification signal and the information signals is less than for co-propagating Raman amplification and information signals.
Preferably the system further comprises at least one erbium amplifier arranged to amplify said first and second information signals, said system being arranged such that one of said information signals undergoes Raman amplification prior to amplification by said erbium amplifier, and the other information signal undergoes Raman amplification after amplification by said erbium amplifier. Of course, the system can include other discrete amplifiers (as distinct from a distributed amplifier), such as discrete Raman amplifiers, Thulium doped fibre amplifiers, or indeed any rare-earthed doped fibre amplifiers. However, discrete amplifiers need not be present if the distributed Raman amplification is sufficiently large.
Preferably, the amplifier and said Raman source are co-located.
In another aspect, the present invention provides a method of providing Raman amplification to a communications system, the system comprising a first optical waveguide suitable for transmitting a first information signal and a second optical waveguide suitable for transmitting a second information signal, the method comprising the steps of: providing an electro magnetic radiation signal into said first waveguide, said signal being suitable for providing Raman amplification of the information signals; and transferring at least a portion of the Raman amplification signal to said second waveguide.
Preferably the system further comprises the step of detecting the presence of the Raman amplification signal in said second waveguide.
Preferably, if the Raman amplification signal is not detected in said second waveguide, the electromagnetic radiation signal is prevented from entering into said first waveguide.
In a further aspect, the present invention provides a computer programme arranged to control a communications system so as to perform the method of providing Raman amplification to a communications system, the system comprising a first optical waveguide suitable for transmitting a first information signal and a second optical waveguide suitable for transmitting a second information signal, the method comprising the steps of: providing an electro magnetic radiation system into said first waveguide, said signal being suitable for providing Raman amplification of the information signals; and transferring at least a portion of the Raman amplification signal to said second waveguide.
The computer programme can be stored on a machine readable medium.